Track a set of active subscriptions. Unsubscribe them all at once.
If a subscription gets unsubscribed, it auto-removes from the set.
Readonly
Start tracking a subscription, so that it is closed with others on +close+.
Unsubscribe all the subscriptions.
Track a set of active subscriptions. Unsubscribe them all at once.
If a subscription gets unsubscribed, it auto-removes from the set.